Cooler

ABSTRACT

A cooler includes: a base to which an object to be cooled is attached; and a fin which is fixed to a surface of the base, the fin including a plurality of graphite sheets laminated in the fin, the graphite sheet having a thermal conductivity higher in an in-plane direction than in an out-plane direction, wherein the fin is fixed to the base at end faces of the plurality of graphite sheets.

CROSS-REFERENCE

This application claims priority to Japanese patent application No. 2017-224021, filed on Nov. 21, 2017, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The technique disclosed herein relates to a cooler. Especially, it relates to a heat sink having a fin fixed to a base.

BACKGROUND

A heat sink (cooler) that uses a graphite sheet in a fin is known. JP 2009-099878 A, JP 2017-084883 A describe such coolers.

SUMMARY

Coolers of JP 2009-099878 A, JP 2017-084883 A each have a graphite sheet bent at a right angle, and a surface of the graphite sheet is fixed to a base. The graphite sheet has a higher thermal conductivity in an in-plane direction than a thermal conductivity in a thickness direction. Thus, the coolers of JP 2009-099878 A, JP 2017-084883 A in which the surface of the graphite sheet is in contact with the base did not efficiently make use of a property of the graphite sheet in regard to thermal conduction at an interface between the base and the fin (graphite sheet).

A cooler disclosed herein may comprise a base to which an object to be cooled is attached; and a fin which is fixed to a surface of the base, the fin including a plurality of graphite sheets laminated in the fin, the graphite sheet having a thermal conductivity higher in an in-plane direction than in an out-plane direction, wherein the fin is fixed to the base at end faces of the plurality of graphite sheets. In the cooler disclosed herein, the direction with the high thermal conductivity of the graphite sheets (the in-plane direction) intersects with the surface of the base at interfaces between the base and the fin. In other words, a standing direction of the fin to the base is substantially parallel to the in-plane direction of the graphite sheets. The cooler disclosed herein has a superior thermal conductivity from the base to the fin compared to the conventional coolers.

In the cooler disclosed herein, an exposed area of the surface of the base (area where the fin is not in contact) may have a water-resistance. Such a cooler is more resistant to deterioration even when fluid coolant is used.

The base of the cooler disclosed herein may include a metal plate; and a thermally conductive layer covering a surface of the metal plate, the thermally conductive layer having a thermal conductivity higher than a thermal conductivity of the metal plate, and the end faces of the plurality of graphite sheets may be fixed to the thermally conductive layer. The end faces of the graphite sheets have better adhesive properties than surfaces of the graphite sheets, and as such, detachment is less likely to occur at thermally conducting portions. With such a structure, the thermal conductivity from the base to the fin is further enhanced.

Further, a plating layer may be provided on each of the end faces of the graphite sheets fixed to the base. By plating the end faces, the adhesive property and a thermal conductive property between the fin and the base is further enhanced.

Details and further improvements of the technique disclosed herein will be described in the following “DETAILED DESCRIPTION”.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a cooler of a first embodiment.

FIG. 2 is a cross-sectional view of the cooler of the first embodiment.

FIG. 3 is a diagram showing an analysis result of the cooler of the first embodiment.

FIG. 4 is a cross-sectional view along a line IV-IV in FIG. 3.

FIG. 5 is a diagram showing an analysis result of a cooler of a comparative example.

FIG. 6 is a cross-sectional view of a cooler of a second embodiment.

FIG. 7 is diagram explaining a manufacturing method of a cooler.

DETAILED DESCRIPTION

Representative, non-limiting examples of the present invention will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide improved cooler, as well as methods for using and manufacturing the same.

Moreover, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described and below-described representative examples, as well as the various independent and dependent claims, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.

All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter.

(First Embodiment) A cooler 2 of a first embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 shows a perspective view of the cooler 2. FIG. 2 shows a cross-sectional view of the cooler 2. The cooler 2 is a heat sink configured to dissipate heat from a heat source 90. The heat source 90 is a cooling target of the cooler 2. The cooler 2 includes a base 20, and a plurality of fins 10. The heat source 90 is attached to a rear surface of the base 20. The plurality of fins 10 is fixed to a front surface of the base 20. The front surface of the base 20 and the fins 10 are exposed to a coolant. The heat from the heat source 90 is transferred from the base 20 to the fins 10, and is dissipated to the coolant from the front surface of the base 20 and surfaces of the fins 10. FIG. 2 shows a cross section that cuts along a plane that is parallel to an XZ plane in a coordinate system of FIG. 1, and transverses the fins 10.

The base 20 includes a metal plate 21 constituted of metal with a high thermal conductivity such as copper or aluminum, a thermal interface material layer 22 provided on a surface of the metal plate 21 on a fin side, and a fixed layer 23. Hereinbelow, for simplicity of explanation, the thermal interface material layer 22 will be termed a TIM layer 22. The TIM layer 22 is constituted of a substance having a higher thermal conductivity than the metal plate 21.

Each fin 10 is configured by laminating a plurality of graphite sheets 12 with sheet binding layers 13. The graphite sheets 12 are sheets with a very high thermal conductivity. The graphite sheets 12 have a higher thermal conductivity in an in-plane direction than a thermal conductivity in a thickness direction. In a coordinate system of FIG. 2, a direction parallel to a YZ plane corresponds to the in-plane direction of the graphite sheets 12, and an X axis direction corresponds to the thickness direction.

The plurality of graphite sheets 12 are laminated so that wide-width surfaces of adjacent graphite sheets 12 are opposed to each other. As clearly shown in FIG. 2, each fin 10 has end faces 121 of the plurality of graphite sheets 12 (surfaces intersecting with the wide-width surfaces) fixed to the TIM layer 22 of the base 20. As aforementioned, the graphite sheets 12 have a higher thermal conductivity in the in-plane direction than a thermal conductivity in an out-plane direction (thickness direction of the graphite sheets 12). In the cooler 2, the direction with the high thermal conductivity of the graphite sheets 12 (in-plane direction) intersects with the front surface of the base 20 at interfaces between the base 20 and the fins 10. In other words, a direction along which the fins 10 extend from the base 20 becomes substantially parallel to the in-plane direction of the graphite sheets 12. Thus, heat is transferred efficiently from the base 20 to the fins 10. Further, in the cooler 2, the fins 10 are fixed to the TIM layer 22 of the front surface of the base 20. This configuration also contributes to efficient heat transfer from the base 20 to the fins 10.

In the cooler 2, two features thereof, namely, the end faces 121 of the graphite sheets 12 being fixed to the base 20, and the fins 10 being fixed to the TIM layer 22 at the front surface of the base 20, contribute significantly to heat transfer from the base 20 to the fins 10.

Other than the above, the cooler 2 of the present embodiment has the following features. An exposed area of a front surface of the TIM layer 22 (area where the fins 10 are not in contact) is covered by the fixed layer 23. The fixed layer 23 confines peripheries of base portions of the fins 10 to support fixation of the fins 10. Further, the fixed layer 23 has water-resistance, and thereby protects the TIM layer 22 from fluid coolant. The fixed layer 23 also has a high thermal conductivity.

In the fins 10 configured of the plurality of graphite sheets 12 and the sheet binding layers 13, at least 30% (30 vol %) of a volume of each of the fins 10 is occupied by the graphite sheets 12. A total average thickness of the TIM layer 22 and the fixed layer 23 is 1 to 500 micron meters. The TIM layer 22 may have the thermal conductivity of 5 W/mk or more, and the fixed layer 23 may have the thermal conductivity of 1 W/mk or more. Fillers that increase the thermal conductive property may be mixed in the TIM layer 22 and the fixed layer 23. A material having a high thermal conductivity is used for the sheet binding layers 13 as well. At least one of the TIM layer 22 and the fixed layer 23 has an insulation property.

The graphite sheets 12 each have a thickness of 1 to 100 micron meters, and the thermal conductivity in the in-plane direction is 200 to 2000 W/mk. 50% or more of an area of the end face of each fin 10 is occupied by the end faces 121 of the graphite sheets 12.

The end faces 121 of the graphite sheets 12 have a superior adhesive property to the surfaces of the graphite sheets 12. Due to this, detachment of the graphite sheets 12 is less likely to occur at joint portions between the fins 10 and the base 20. In other words, the fins 10 strengthen the joint portions with the base 20 by having their end faces fixed to the base 20.

An analysis result of performances of the cooler 2 of the embodiment will be described. FIG. 3 shows a plan view of a cooler model 102 (cooler model 102 of the embodiment) used in an analysis, and FIG. 4 shows a cross-sectional view along a line IV-IV in FIG. 3. The cooler model 102 used in the analysis is basically identical to the cooler 2 of FIG. 1. Heat generating components 90 a, 90 b are attached to a rear side of a base 120 of the cooler model 102, and a plurality of fins 110 is fixed to a front surface of the base 120. The base 120 includes a metal plate 129, a TIM layer 122, and a fixed layer 123.

FIG. 5 shows a plan view of a cooler model 902 of a comparative example. The fins 110 in the cooler model 102 of the embodiment are laminates of graphite sheets, whereas fins 910 of the cooler model 902 of the comparative example are round columns constituted of aluminum. In the cooler model 102 of the embodiment, a thickness of the fixed layer 123 is set to 0.8 mm, and its thermal conductivity in a thickness direction is set to 50 W/mk. On the other hand, in the cooler model 902 of the comparative example, a thickness of a fixed layer is set to 0.8 mm, and its thermal conductivity in a thickness direction is set to 200 W/mk. In FIGS. 3 and 5, a temperature distribution is indicated on the heat generating components 90 a, 90 b by gradation of hatching therein. Dark hatching indicates a lower temperature. In comparing FIGS. 3 and 5, it can be understood that with left upper portions (P1, P2 in the drawings) of the heat generating component 90 a on an upper side in the drawings, the cooler model 102 of the embodiment has a lower temperature as compared to the cooler model 902 of the comparative example. Further, in the heat generating component 90 b on a lower side in the drawings, a maximum temperature is 77.9° C. in the case of the cooler model 902 of the comparative example, whereas a maximum temperature is 77.1° C. in the case of the cooler model 102 of the embodiment. It has been found that the cooler model 102 of the embodiment has a higher cooling performance than the cooler model 902 of the comparative example.

(Second Embodiment) FIG. 6 shows a cross-sectional view of a cooler 2 a of a second embodiment. In the cooler 2 a of the second embodiment, a metal plating layer 15 is provided at the end faces 121 of the fins 10 on their sides fixed to the base 20. The fins 10 have the end faces 121 provided with the metal plating layer 15 fixed to the TIM layer 22 of the base 20. By providing the metal plating layer 15, the adhesive property of the fins 10 is increased, and the thermal conductive property between the base 20 and the fins 10 is improved.

An example of a method of manufacturing a cooler of the embodiment will be described with reference to FIG. 7. A graphite sheet being a material of a fin is rolled up in a roll. The graphite sheet is drawn out from a sheet roll 31, and a binding agent 33 is applied to a surface of the sheet using an applicator 32 (A in FIG. 7). Thermosetting resins such as epoxy resin may be used as the binding agent, however, no limitation is made hereto. Those with high chemical stability among thermoplastic resins, such as polyimide resin, may be used as the binding agent. Fillers with high thermal conductivity such as alumina may be mixed in the binding agent.

In order to increase adhesive property and thermal conductive property, surface finishing, end surface finishing, or plating may be performed on the graphite sheet before applying the binding agent. A graphite sheet on which a metal foil or a metal mesh is laminated may be used.

The graphite sheet onto which the binding agent has been applied on the surface thereof is rolled up again into a roll, and is cut into a length of the fin (B in FIG. 7). When the binding agent 33 solidifies, it becomes the sheet binding layers 13 of FIG. 1. A graphite sheet roll 34 may be used as the fin. Alternatively, the roll 34 provided with a metal plating layer 35 at its end face (end face to be fixed to a base) may be used as the fin (C in FIG. 7). Alternatively, the roll 34 that is vertically cut (semi-circular roll 36) may be used as the fin (D in FIG. 7). Moreover, a metal plating layer 37 may be provided at an end face of the semi-circular roll 36 (end face to be fixed to the base) (E in FIG. 7).

FIG. 7 indicates the method of manufacturing the round columnar (or semi-circle columnar) fin, however, flat graphite sheets onto which the binding agent has been applied may be laminated, and may be cut into suitable sizes to obtain square-columnar fins.

An example of the manufacturing process of the cooler is as follows. A plurality of the fins manufactured by the above manufacturing method are aligned using a vibration alignment machine. A heat transfer material to be the TIM layer 22 is applied or adhered to the surface of the metal plate 21 configuring the base 20, and is temporarily attached to the metal plate 21 by heating. The plurality of fins that were arranged is heated and pressed onto the temporarily-attached heat transfer material, and the fins are thereby temporarily fixed to the heat transfer material (TIM layer). A material that is to be a fixed layer is poured to base portions of the fins by transfer molding, and the cooler is completed by solidifying the material.

The heat transfer material used for the TIM layer is not particularly limited, however, it may have the insulation property. The heat transfer material used for the TIM layer may have the thermal conductivity of 5 W/mk or more. In considering the chemical stability of the material, a material mainly constituted of thermosetting resin may be used.

In the surface finishing performed on end faces of the fins to be fixed to the base, a metal layer may be formed on the end faces by sputtering or plating. In order to increase the binding force with the metal, the sputtering or plating may be performed after having the end faces subjected to chemical etching.

The material used for the fixed layer is not particularly limited, however, it may be adhesive-based thermosetting composite resin having the thermal conductivity of at least 1 w/mk or more. Heat reversible resins may be used. A thickness of the fixed layer may become thinner at portions farther away from the fins. Further, the material used for the fixed layer (adhesive-based thermosetting composite resin) has the water-resistance.

The shape of the fins is not limited to circular column or square column, and may be U-shaped, corrugated, oval, or combinations of such shapes.

The embodiments have been described in detail above, however, these are mere exemplary indications and thus do not limit the scope of the claims. The technique described in the claims includes modifications and variations of the specific examples presented above. Further, the technical features described in the description and the drawings may technically be useful alone or in various combinations, and are not limited to the combinations as originally claimed. Further, the technique described in the description and the drawings may concurrently achieve a plurality of aims, and technical significance thereof resides in achieving any one of such aims. 

What is claimed is:
 1. A cooler comprising: a base to which an object to be cooled is attached; and a fin which is fixed to a surface of the base, the fin including a plurality of graphite sheets laminated in the fin, the graphite sheet having a thermal conductivity higher in an in-plane direction than in an out-plane direction, wherein the fin is fixed to the base at end faces of the plurality of graphite sheets.
 2. The cooler as in claim 1, wherein an exposed area of the surface of the base has a water-resistance.
 3. The cooler as in claim 1, wherein the base comprises: a metal plate; and a thermally conductive layer covering a surface of the metal plate, the thermally conductive layer having a thermal conductivity higher than a thermal conductivity of the metal plate, and the end faces of the plurality of graphite sheets are fixed to the thermally conductive layer.
 4. The cooler as in claim 1, wherein a plating layer is provided on each of the end faces fixed to the base. 5 The cooler as in claim 1 wherein the in-plane direction of the graphite sheets and a standing direction of the fin are substantially parallel to each other in a state where the fin is fixed to the base. 